Review checklist for kvm patches

  1. The patch must follow Linux kernel coding style and Submitting patches: the essential guide to getting your code into the kernel.

  2. Patches should be against kvm.git master or next branches.

  3. If the patch introduces or modifies a new userspace API: - the API must be documented in The Definitive KVM (Kernel-based Virtual Machine) API Documentation - the API must be discoverable using KVM_CHECK_EXTENSION

  4. New state must include support for save/restore.

  5. New features must default to off (userspace should explicitly request them). Performance improvements can and should default to on.

  6. New cpu features should be exposed via KVM_GET_SUPPORTED_CPUID2, or its equivalent for non-x86 architectures

  7. The feature should be testable (see below).

  8. Changes should be vendor neutral when possible. Changes to common code are better than duplicating changes to vendor code.

  9. Similarly, prefer changes to arch independent code than to arch dependent code.

  10. User/kernel interfaces and guest/host interfaces must be 64-bit clean (all variables and sizes naturally aligned on 64-bit; use specific types only - u64 rather than ulong).

  11. New guest visible features must either be documented in a hardware manual or be accompanied by documentation.

Testing of KVM code

All features contributed to KVM, and in many cases bugfixes too, should be accompanied by some kind of tests and/or enablement in open source guests and VMMs. KVM is covered by multiple test suites:

Selftests

These are low level tests that allow granular testing of kernel APIs. This includes API failure scenarios, invoking APIs after specific guest instructions, and testing multiple calls to KVM_CREATE_VM within a single test. They are included in the kernel tree at tools/testing/selftests/kvm.

kvm-unit-tests

A collection of small guests that test CPU and emulated device features from a guest’s perspective. They run under QEMU or kvmtool, and are generally not KVM-specific: they can be run with any accelerator that QEMU support or even on bare metal, making it possible to compare behavior across hypervisors and processor families.

Functional test suites

Various sets of functional tests exist, such as QEMU’s tests/functional suite and avocado-vt. These typically involve running a full operating system in a virtual machine.

The best testing approach depends on the feature’s complexity and operation. Here are some examples and guidelines:

New instructions (no new registers or APIs)

The corresponding CPU features (if applicable) should be made available in QEMU. If the instructions require emulation support or other code in KVM, it is worth adding coverage to kvm-unit-tests or selftests; the latter can be a better choice if the instructions relate to an API that already has good selftest coverage.

New hardware features (new registers, no new APIs)

These should be tested via kvm-unit-tests; this more or less implies supporting them in QEMU and/or kvmtool. In some cases selftests can be used instead, similar to the previous case, or specifically to test corner cases in guest state save/restore.

Bug fixes and performance improvements

These usually do not introduce new APIs, but it’s worth sharing any benchmarks and tests that will validate your contribution, ideally in the form of regression tests. Tests and benchmarks can be included in either kvm-unit-tests or selftests, depending on the specifics of your change. Selftests are especially useful for regression tests because they are included directly in Linux’s tree.

Large scale internal changes

While it’s difficult to provide a single policy, you should ensure that the changed code is covered by either kvm-unit-tests or selftests. In some cases the affected code is run for any guests and functional tests suffice. Explain your testing process in the cover letter, as that can help identify gaps in existing test suites.

New APIs

It is important to demonstrate your use case. This can be as simple as explaining that the feature is already in use on bare metal, or it can be a proof-of-concept implementation in userspace. The latter need not be open source, though that is of course preferrable for easier testing. Selftests should test corner cases of the APIs, and should also cover basic host and guest operation if no open source VMM uses the feature.

Bigger features, usually spanning host and guest

These should be supported by Linux guests, with limited exceptions for Hyper-V features that are testable on Windows guests. It is strongly suggested that the feature be usable with an open source host VMM, such as at least one of QEMU or crosvm, and guest firmware. Selftests should test at least API error cases. Guest operation can be covered by either selftests of kvm-unit-tests (this is especially important for paravirtualized and Windows-only features). Strong selftest coverage can also be a replacement for implementation in an open source VMM, but this is generally not recommended.

Following the above suggestions for testing in selftests and kvm-unit-tests will make it easier for the maintainers to review and accept your code. In fact, even before you contribute your changes upstream it will make it easier for you to develop for KVM.

Of course, the KVM maintainers reserve the right to require more tests, though they may also waive the requirement from time to time.